1. Field of the Invention
This invention relates to a method of improving the image quality of a subtraction image in subtraction processing of radiation images, and an apparatus for carrying out the method. This invention particularly relates to a method of improving the image quality of a subtraction image in digital subtraction processing of radiation images conducted by use of stimulable phosphor sheets, and an apparatus for carrying out the method.
2. Description of the Prior Art
Conventionally, a digital subtraction processing method is used for processig radiation images. In the method, two radiation images recorded under different conditions are photoelectrically read out to obtain digital image signals, which are then subjected to subtraction processing with respect to the corresponding picture elements of the images, to thereby obtain a difference signal for forming an image of a specific structure contained in the radiation images. The method makes it possible to reproduce a radiation image of only the specific structure by use of the signal thus obtained.
Basically, subtraction processing methods are classified into the temporal (time difference) subtraction processing type and the energy subtraction processig type. In the former method, the image of a specific structure is extracted by subtracting the image signal of a radiation image obtained without injection of contrast media from the image signal of a radiation image in which the image of the specific structure is enhanced by the injection of contrast media. In the latter method, an object is exposed to radiations of differing energy distributions to obtain two radiation images each containing the images of a specific structure recorded on the basis of the intrinsic radiation energy absorption characteristics of the specific structure. The image signals of the two radiation images are then weighted appropriately, when necessary, and subjected to subtraction to extract the image of the specific structure.
Since the subtraction processing is extremely effective for diagnostic purposes in image processing of medical X-ray photographs, it has recently attracted much attention, and research has continued to develop improved methods employing electronic technology. In particular, there is the processing technique called digital subtraction processing, or usually just digital radiography (abbreviated to "DR").
A novel digital subtraction processing method has been proposed, for example, in Japanese Unexamined Patent Publication No. 58 (1983)-163340. The method comprises the steps of (i) using two or more stimulable phosphor sheets exhibiting an extremely wide latitude of exposure to a radiation, (ii) exposing the stimulable phosphor sheets to radiation passig through the same object under different conditions to store radiation images of the object in the stimulable phosphor sheets, image information of a specific structure being different between the radiation images, (iii) detecting the radiation images by scanning with stimulating rays to obtain digital image signals, and (iv) conducting digital subtraction processing by use of the digital image signals. The stimulable phosphor sheets comprise a stimulable phosphor which is able to store a part of the radiation energy when exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, and then emits light in proportion to the stored energy of the radiation when exposed to stimulating rays such as visible light, as disclosed for example in U.S. Pat. No. 4,258,264. The stimulable phosphor sheets exhibit an extremely wide latitude of exposure and a markedly high resolving power. Therefore, when the digital subtracting processing is conducted by use of the radiation images stored in the stimulable phosphor sheets, it is possible to obtain a radiation image having an improved image quality, particularly a high diagnostic efficiency and accuracy, regardless of the amount of the radiation of which the stimulable phosphor sheets are exposed.
However, when radiation images of an object which include a bone image and a soft tissue image are recorded on two stimulable phosphor sheets and energy subtraction processing is conducted to obtain a subtraction image wherein the bone image or the soft tissue image is erased to extract only the soft tissue image or the bone image, since the extracted image signal of the soft tissue or bone is a difference signal generatedby the subtraction processing, the signal-to-noise ratio thereof is low, and the image quality of the subtraction image wherein the soft tissue image or the bone image is extracted is degraded.